Method and means for balancing sensitivity of dual tubes



Jan. 23, 1951 P, GLASS 2,539,127

METHOD AND MEANS FOR BALANCING SENSITIVITY OF DUAL TUBES 2 Sheets-Sheet1 Filed Dec. 11, 1948 Jan. 23, 1951 P GLASS 2,539,127

METHOD AND MEANS FOR BALANCING SENSITIVITY 0F DUAL TUBES Filed Dec. 11,1948 2 Sheets-Sheet 2 FIG;

P GLASS 1N VEN TOR.

Patented'Jan. 23, 1951 METHOD AND MEANS FOR BALANCING SENSITIVITY OFDUAL TUBES Paul Glass, Chicago, Ill, assignor to Askania RegulatorCompany, Chicago, Ill.,- a corporation of Illinois Application December11, 1948, Serial No. 64,757

12 Claims.

This application is continuation in part of my copending application forUnited States patent, Serial No. 489,207, filed May 31, 1943,- nowPatent No; 2,476,657, dated July 19, 1949. Whereas that applicationrelates to a motor control circuit including dual tubes, selectivelyrendered conductive to select direction of operation of the controlledmotor, the present invention relates to balancing sensitivity of dualtubes, several forms of the invention being disclosed in the notedcopending patent application.

Dual tubes with a common grid circuit are well knownand used in atremendous number of different circuit arrangements, such as thosewherein selection of one of two circuits for energiza- .tion isaccomplished by signals applied to the tubes, so that the tubes act as aswitch or wherein degree of energization of two circuits depends onsignals applied to the respective tubes, wherein the tubes act asvariable resistances, or wherein two parallel connected tubes are tocontrol simultaneous energization of two circuits in the manner of aswitch, or to simultaneously 'vary degree of energization of thecircuits that they respectively control, by application to the tubes ofa common signal, or by application to the'respective tubes of differentsignals. In any such situation, it is highly desirable that the pairedtubes be of equal sensitivity, so that application of a common signalvoltage, application of equal signal voltages to the respective tubes atdifferent times, and so forth, will produce exactly similar operation ofthe two tubes. Commercially available tubes, however, vary incharacteristics of sensitivity from tube to tube, and consequentlyoperation of dual tube controlled circuits has been-much more difficultin practice than in theory. Such expedients as careful matching of tubesfor close sensitivity similarity, and independent adjustment of biasvoltages have been resorted to. The former expedient requires a tediousand expensive matching procedure, the latter, while being fairlysatisfactory so far as sensitivity equalizing per se is concerned, tendsto upset the grid circuit conditions by changing the response of one orthe other, or both tubes to a given value of signal.

One object of the present invention is to provide simple and effectivemethod and means permitting sensitivity equalization of dual tubes,which does not involve adjustment of the tubecontrolling bias, and whichpermits exact balance of response of the tubes to a given value ofsignal.

Another object is the provision of such method 2 and means capable ofuse with a dual tube cominon grid circuit of practically anyarrangement, and usable with various types of signal voltages, orvariable amplitude direct or alternating current signal voltages.

The invention is based on the concept of simultaneously, equally andoppositely varying the sensitivity of two tubes having a common gridcircuit, by applying between the control grids a sensitivity biasvoltage, so that adjustment of such voltage increases bias on one tubeand simultaneously decreases that of the other, in the manner ofswinging the respective grid voltages in opposite directions about themain bias voltage that primarily selects response of the two tubes to agiven signal.

In the accompanying drawings,

Figs. 1, 4 to 6, 9 and 10 are schematics showing various selectedembodiments of the invention applied to different grid circuits of dualtubes, and illustrating the flexibility and great extent of possibleapplications of the invention.

Figs. 2 and 3 are vector diagrams of operation of the form of inventionof Fig. 1.

Figs. '7 and 8 are voltage graphs indicating operation of the inventionas applied in Fig. 6.

In the present application the term dual tubes is intended to apply totubes having a common control grid circuit, and operating to control oneor more output circuits. The term includes, but is not limited to suchtubes as comprise dual sets of elements in a single envelope, and itapplies equally to entirely separate tubes.

The drawings present only a few of the many fields of use andarrangements of the invention, and are to be considered as being purelyfor illustrative disclosure and example, and not as limiting theinvention to the specific forms therein shown.

Figs. 1 to 6 represent grid circuits disclosed by the noted Patent No.2,476,657, with elimination or simplification of parts of the circuitsdisclosed by such application that are not necessary to fullunderstanding of the present invention.

Describing the drawings in detail, and first referring to Fig. 1, tubesl5 and I6 exemplify a pair of dual tubes, respectively controllingcircuits H and I8 that may include loads, represented by resistances l9andZEl, it being assumed that such circuits are to be selectivelyenergized or both deenergized. The anode circuits l1 and N3 of the tubesare supplied with alternating voltage in phase agreement through atransformer 2|, the secondary of which is connected in the 3 commonanode return of the tubes, and the primary energized by an alternatingcurrent source 22. To prevent conduction of the tubes during positivehalf cycles of anode voltage, a suppressing bias of adjustable amplitudeand phase is applied to the control grids 23 across a resistance 2iconnected in the common return of the grid circuit. Signal potentials ofopposite sense are applied to the control grids of the tubes across gridreturn resistances 25 by condensers 26 which connect correspondingterminals of such resistances, and the control grids to opposite ends ofthe secondary of a signal input transformer 21, a condenser 23 beingshunted across such secondary.

The variable amplitude and phase suppressing voltage applied to the twocontrol grids in phase agreement is supplied across the resistance 24through a potentiometer 29 and a phase shifting network 30 from thesecondary of a transformer 3 I, the primary of which is energized by thesame alternating current source 22 that supplies the anode circuits ofthe tubes. The alternating suppressing voltage applied across theresistance 24 is, by means of potentiometer 29, adjusted to preventconduction by either tube 23 in the absence of a signal voltage whensuch voltage is substantially in phase opposition to the anode voltage.The phase shifter 3e may be so adjusted that the suppressing voltageapplied to the control grids of the two tubes leads the anode voltage bysomewhat less than 180, to overcome a dead zone of the tubes and renderthem sensitive to a very small signal.

In this form of circuit, the signal may take the form of a variableamplitude voltage applied in opposite phase to the two control grids,and in such phase relation to the anode andsuppressing voltagesvariation in its amplitude serves to advance and retard phases of theresultant controlling voltages applied to the grids of the respectivetubes depending on the phase of the signal voltage, so that one tube isrendered conductive while the other is maintained non-conductive. Whilea variety of signal circuits may be used for accomplishing such results,Fig. 1 shows a circuit disclosed by the noted copending application.

In Fig. 1, a bridge circuit 32 is energized by a transformer 33 havingits primary energized from the source 22 so that phase of the signalvoltage may properly be related to those of the tubeenergizing circuits.The bridge 33 has a movable contact 34 by which a signal of reversiblephase and variable amplitude may be generated. Such voltage is appliedto the primary of the signal transformer 21 through a potentiometer 25and a phase shifter 36. Ihis phase shifter is arranged to displace thesignal voltage by substantially 90 with reference to the anode voltageof the tubes. By this arrangement, variation of the output oi bridge 32by variation of contact 35 serves to advance and retard the respectivegrid voltages, identity being determined by the phase of such signalvoltage, as indicated above, to render conductive the tube of which thegrid voltage is advanced.

In order to equalize sensitivity of the two tubes so thatthey'respectively become conductive at equal degrees of advancing phaseshift of the respective controlling grid voltage, resulting from therespective shifting effect of signal voltages of equal but oppositephase, in spite of variance between the firing response of therespective tubes to magnitude of control grid voltage phase advance,means are provided for applying an auxiliary inter-grid bias forproducing equal and opposite sensitivity variance compensating shifts inphase between the resultant grid voltages in the two grid circuits. InFig. '1 this takes the 5 form of a device for injecting in the two gridcircuits supplemental alternating current bias voltages that are ofopposite phase relative to each other, and displaced in phase a variabledegree with respect to the suppressing voltage applied across theresistance 24.

Such means comprises a center tapped potentiometer e7 supplied from thesource 22 by the secondary of the suppressing voltage supplyingtransformer 3i, through a limiting resistance 38. Variation of themovable contact of the potentiometer 37 selects the relative directionsof grid voltage phase shifts introduced by the supplementalphase-shifting bias voltages, and magnitude of such variations thedegrees of such shifts. By such variations, adjustment may be made of"le amplitudes of signal voltages introduced by the transformer 22',which renders the respective tubes conductive.

The operation of the equilizer arrangement may b explained by referenceto thevector diagrams of Figs. 2 and 3. Vector A represents the line, ortube anode voltage, and a represents the suppressing bias voltageapplied across resistance 24, which is common to the two grid circuits.The phase relation of vector a is fixed by phase shifter 30, and thisvoltage leads the anode voltage by (180-e1l), degrees. If it is supposedthat the phase shift angles necessary to render tubes 23 just conductiveareel and e2, r respectively, then the pre-phase shift eil would not besuflicient to fire one tube, but would be ample to fir the other. Thesupplemental equalizing voltage introduced from equalizer 31 is shown bythe vectors b and b in Figs. 2 and 3 respectively, and it will be notedthat these vectors have opposite phase relation, 2) being in phase withthe anode voltage, while I) is opposite in phase to the anode voltage.The resultant alternating biasing, voltages for the two grid circuitsare now represented by the vectors cl for one tube and 02 for the other,and these vectors have the proper phase angle to adjust both tubes sothat they become conductive at equal signal ValuLs.

In practice, the amount of equalizing voltage necessary to equalize thetwo tubes is small compared with the common suppressing voltagedeveloped across the resistance 24. It is not necessary that theequalizing voltages be in phase and opposite to the phase of the anodevoltages, but preferably they should be out of phase with thesuppressing voltage across resistance H.

Adjustment of sensitivity of the two tubes readily can be accomplishedby short circuiting the input terminals of the transformer 21, andadjustment of the equalizing potentiometer 31 until both tubes becomeconductive at the same instant upon advancement of the phase of thesuppressing voltage by variation of phase shifter 30.

Fig. 4 shows a variation in the equalizer arrangement, and alsoillustrates an arrangement for controlling by a direct current signal apair of dual tubes supplied with alternating anode and suppressingvoltages in the manner of Fig. l. The equalizer itself is the same as inFig. 1, being a center tapped potentiometer 4B, supplied by atransformer 4i, and having its variablicontact and center tap connectedin series in the circuit. In Fig. 3 the equalizer is connected in shownin Figs. 2 and 3.

the inputto the phase shifter 42, rather than in the output. Thisarrangement has the advantage that the equalizing voltages are shiftedin phase by the phase shifter 42, and hav a phase angle displaced inphase by substantially 90 with reference to the equalizing voltages b,With this phase angle, smaller equalizing voltages are required to shiftthe resultant grid voltages from the vector at to the vector 0, and theresultingvectors cl and 02 in Figs. 2 and 3 are substantially equal inamplitude.

For control of tubes 43 by a direct current signal of reversiblepolarity and variable amplitude, such signals may be impressed from asource 44 across the grid circuits of such tubes through a potentiometert5 and series resistances or inductances as which prevent the directcurrent signal source from influencing the alternating voltages appliedto the grid circuits through the transformer 41;

By the arrangement of Fig. 4, a direct current signal of one polaritywill cause conduction of the tube while a signal of opposite polaritywill cause conduction of the other tube. The effective output currentwill vary in accordance with the amplitude of the direct current signal,and operation is not dependent upon presence of an alternating voltageapplied through potenticmcter 48, although the system will respondequally well to a variable amplitude alternating voltage of proper phaserelation, as explained relative to Fig. 1, applied through thepoten--tiometer 48.

While the previous circuits have been arranged to apply equalizingvoltages to the grid circuits of both tubes, it is possible toaccomplish equalization in a simple and satisfactory manner by applyingequalizing voltage between the grids by means of a voltage sourceconnected in one grid circuit. An arrangement of this kind is shown byFig. 5, the grid circuit arrangement therein, as in Fig. 4, being suchthat either a direct or an alternating voltage of variable amplitude maybe used to control the dual tubes 5%.

Direct current signal voltages are applied across resistances El, 52 inseries, and an equalizing potentiQmetLr 53 is connected in series withthe grid circuit of one of the tubes 50, whereby an alternating currentequalizing voltage of reversible phase and variable amplitude, both ofwhich areselected by the position of they movable contact of thepotentiometer relative to its center tap, may be applied to the gridcircuit in which the potentiometer is connected. The potenti o'rneter isshown as being supplied from a sec- .ondary winding 54, of a transformer55, separated from the secondary winding 56 that supplies thesuppressing voltage across the resistance 24, but may be supplied as inFigs. 1 and 4, .or in other suitable manner. 'may b included in thesupply circuit to poten- A phase shifter tiometer 53, as indicated at51, so that the equalizing voltage can be made in phase quadrature withthe suppressing voltage, or to have any plied between the grids.

In Fig. 6 is shown a control circuit for selecting, by means of a directcurrent signal of revers ible polarity and variable amplitudeconductivity of two tubes 60, which respectively control output circuits6| having a common return 82, and energized by direct connection with analternating current source 63. The direct current signal is impressedacross resistance 94 in series with the center tap and variable contactof the resistances of an equalizing potentiometer 65, to which directcurrent is supplied from a source represented by battery 66. It will beunderstood that the equalizer potentiometer 65 suppliesequalizingpotentials of opposite polarity to the grid circuits of the tubes 56,and that such potentials may be varied in amplitude and reversed inpolarity by adjusting the contact of the potentiometer. The

' secondary winding of a suppressing bias trans- B1 and potentiometer68.

l of the signal.

former Bl, energized by the source 63, is connected in series in thecommon grid lead for the two tubes, and supplies a suppressing voltagein 180 phase opposition to the anode voltage, and of sufficientamplitude to prevent conduction of the tubes in absence of a signalvoltage.

A variable direct current voltage is also introduced in the common gridlead for the two tubes by a potentiometer 63 supplied from a suitablesourcerepresented by the battery 69.

In Fig. '7, the curve A represents the potentials applied to the platesof tubes 69 in Fig. 6. The dotted curve a represents the commonalternating suppressing voltage applied to the grid circuits of the twotubesfrom the secondary of transformer ET. The line 12 parallel to thezero line represents the positive potential applied to each grid fromthe potentiometer 68, and the solid curve 0 represents the resultantpotential applied to each grid of tubes (ill from transformer It isclear that the resultant effect of the direct current bias frompotentiometer 68 is to shift the alternating suppressing voltage curve aupwardly to the position 0, and the direct current biasing voltage isadjusted until this curve just fails to touch the criticalcharacteristic curve (1. In this way, the so called dead zone of thetubes is reduced, and a very small signal voltage will serve to causeone tube to fire, depending on the polarity If the two tubes haveslightly different characteristics, adjustment of the contact onequalizer 65 will secure operation of the two tubes in response to equalsignal voltages. The effect of the equalizer adjustment is illustratedin Fig. 8 where curves ci and c2 represent the individual grid voltagecurves which are required in order to just fail to touch the respectivecritical grid characteristics. These characteristic curves as well asthe direct current bias line are omitted from Fig. 8 for the sake ofclearness, and it will be understood that the separation between thecurves 0! and 02 is greatly exaggerated.

An incoming signal which causes the grid of one tube 60 to gomorepositive will cause this tube to conduct to energize one circuit 6i,while an incoming signa1 which causes the grid of the other tube to gomore positive will energize the other circuit 6|. In either case, theincoming signal causes the grid of the inoperative tube to become lesspositive and thereby insures against operation of both tubes.

Fig. 9 discloses a difierent type of signal circuit wherein phase of asignal voltage selects one of dual tubes 10 to be rendered conductive,and amplitude of such voltage determines amplitude of voltages of thetube are supplied in phase agreem'ent through a transformer I l, thesecondary of which is connected in the common return of the two outputcircuits i2, and the primary of which is energized by an alternatingcurrent source Bias to prevent conduction of the tubes in absence of asignal may be applied by a potentiometer 14 connected in the common gridlead and energized by a suitable source 15. An alternating signalvoltageis applied across series connected resistances l6, connectedbetween the grid circuits of the respective tubes from the seriesopposedconnected windings ll of a differential transformer 18, the primary 139of which is energized by the source 73. As is well known," output of thesecondary of such a transformer depends onrelative degrees of couplingof the two secondary windings i? to the primary 19. Such relativecoupling is varaible by a movable core 88 that may be actuated by amechanical control device 8|. Presence of an output secondary voltagedepends on unequal coupling between the windings 11 and the primary 19,the identity of the closer coupled windings determining the phase of theoutput, which is of reversible but otherwise fixed phase, and theamplitude of the voltage depending on the degree of unbalance producedby the unequal coupling.

In Fig. 9, equalization of the sensitivity of the two tubes isaccomplished by connecting in series in the output circuit or thetransformer 18, the secondary of a transformer 82, the primary of whichis energized from source '13 through a phase-reversing and amplitudeselecting center tapped potentiometer 83. This arrangement permitsapplication between the grids of the respective tubes is an alternatingpotential of reversible phase and variable amplitude by means of whichmay be equalized response of the tubes to signals of equal amplitude andopposite phase, generated by the transformer 18 and applied to the gridsacross the resistances 16.

In 10 is shown application of the invention to a bridge type signalcircuit forming the input of a position control circuit, by means ofwhich direction of operation of a motor 85 may be selected, and degreeof operation of the motor is made proportional to magnitude of a signalby a signal-balancing feedback arrangement operated by the motor.

The bridge circuit comprises a pair of potentiometer resistances 85connected across a source of current 82 that may be either direct oralternating, and the voltages of the respective variable potentiometercontacts are applied to the control grids of the dual tubes 83. Thevariable contact of one potentiometer =86 may constitute a signalvo'tage regulator, and may be driven by a mechanical controller 8%,while the variable contact of the other potentiometer is movable by thecontrolled motor 85 to balance the bridge upon completion by the motorof an operation proportional in degree to magnitude of a signal voltageproduced by moving the signal contact to unbalance the bridge.

In applying the invention to this circuit, an equalizing potentiometer90, energized by a suitable source i, the character of which depends onthat of source 81, has its center tap and variable constant connected inseries across a part of one of the bridge potentiometer resistances 86,shown as the signal potentiometer. By adjustment of the movable contactof the potentiometer, opposite potentials, of reversible phase andvariable magnitude are applied to the grids of the respecsis tive tubes88, and consequently compensation may be made for differencesinsensitivity characteristics of the tubes.

Several types of circuit that may be controlled by a dual tube stage,wherein the tubes are compensated in accordance with the presentinvention .to balance their sensitivity, are disclosed by myabove-identified copending application. In that application the tubescorresponding to the tubes of Fig. l are gas discharge tubes,controlling a motor and the grid circuit therein disclosed is providedwith auxiliary pulse-creating means .for providing an anti-hunt drive ofthe motor.

It will be understood that in any of the circuits herein disclose-d,where the tubes are used to control variable voltage or current outputcircuits, and the system is so arranged that the tubes are operated toproduce such variable outputs, the tubes may be either of gas dischargetype, as indicated by the dots enclosed in the tube envelopes of Figs. 1and 6, or, if the circuit is designed for such tubes they maybe of highvacuum voltage-amplifying or current-controlling type. As set forth inthe parent application, in the circuits relating to control of the dualtubes by phase shift of grid voltage through variation of amplitude of asignal voltage having a fixed phase relation to the suppressing bias,and Wherein the dual tubes are of gas discharge type, the suppressingvoltages are substantially in excess of those necessary to preventfiring of the tubes, this safety measure against unintended firing beingpermitted by the provision of means for combining with the suppressingvoltage a variable amplitude signal voltage having a fixed phaserelation tothe suppressing .bias, and therefore capable of a readily andaccurately controllable phase shift of the resultant grid voltages bythe simple expedient of varying its amplitude. This arrangement is veryvaluable, especially in the combination with adjustable auxiliary thirdbiasing means for adjusting phase of the resultant grid voltages toreduce "the dead zone of the tubes to minimum.

It will be appreciated from the foregoing that many variations of theinvention not disclosed by the drawings and specification may beresorted to, and the invention may be applied to a great many circuitsother than those described. While the disclosure of the invention islimited to application in the primary input circuit of a stagecontaining dual tubes, it is obvious that it can be applied equally wellin intermediate amplifying, inverting or other stages, and also to poweror output stages. Accordingly it is to be understood that the scope ofthe invention and that of the protection accorded to it is determined bythe claims rather than by the disclosure.

I claim:

1. In a control circuit that includes a pair of gaseous discharge tubes,a source of alternating current for energizing the anode circuits ofsaid tubes by voltages having the same phase relation, means forsupplying to the grid circuits of said tubes alternating suppressingvoltages of the same phase and amplitude to normally prevent firing ofsaid tubes, and means for supplying to the grid circuits of said tubessignal voltages having opposite variable tube-firing characteristicscontrolling the firing of said tubes; an arrangement for compensatingvariance between the firing responses of such tubes to magnitudes oftube-firing characteristics of such signal voltages, comprising meansfor introducing in said respective grid circuits supplementalalternating voltages for firing the respective tubes at equal magnitudesof tube-firing characteristics of such signal voltages, said means beingarranged to supply such supplemental voltages in opposite phase anddisplaced in phase with respect to said suppressing voltages, andadjustable to permit selection of sense 01' such phase displacement andamplitude of such supplemental voltage.

2. A control circuit comprising, in combination, a pair of gaseousdischarge tubes, a source of alternating current for energizing theanode circuits of said tubes by voltages having the same phase relation,means for supplying to the grid circuits or" said tubes alternatingsuppressing voltages having suitable phase and amplitude values normallyto prevent firing of said tubes, and adjustable means arranged iorequally and oppositely varying the phase relation between suchrespective suppressing voltages. v

3. A control circuit according to claim 2 wherein said phase adjustingmeans comprises means for advancing the phase of one grid suppressingand arranged to apply between said grid circuits a potential havingcharacteristics respectively to aid and oppose tube-firing effects ofsignal voltages applied to the grids of the respective tubes by saidcontrol grid circuits according to sense of such potential. I

'7. A method of balancing sensitivity of dual electron tubes thatcomprises applying between the grids of said tubes of a potential thataids and opposes signal voltages applied to the grids of the respectivetubes.

8. A method of balancing sensitivity of biased dual electron tubes thatcomprises applying bevoltage and for simultaneously retarding the phaseof the other grid suppressing voltage, and vice versa.

4. A control circuit comprising, in combination, a pair of gaseousdischarge tubes, a source of alternating current for energizing theanode circuits or said tubes by voltages having the same phase relation,means for supplying to the grid circuits of said tubes alternatingsuppressing voltages of the same phase and amplitude to normally preventfiring of said tubes, means for supplying to the grid circuits of saidtubes signal voltages for selectively controlling the firing one or theother of said tubes, and means for introducing into the grid circuit ofat least one of said tubes a supplemental alternating voltage of thesame frequency as the suppressing voltage, said means being so arrangedthat such supplemental voltage is reversible in phase and variable inamplitude whereby said tubes may be conditioned for firingat the samesignal values.

5. The combination with interconnected control grid circuits forcontrolling operation of dual tubes, and means for impressing on thecontrol grids of such tubes signal voltages to control tube operation,of means for compensating variance between sensitivities of therespective tubes comprising means for applying between the grids of saidtube a potential of selective sense and having oppositely effectivetube-firing characteristic-altering effects that respectively aid andoppose signal voltages applied to the respective grids according tosense of such potential.

6. The combination with control grid circuits for controlling operationof dual tubes, said circuits being interconnected by a grid-cathode leadincluding grid bias means, of means for compensating variance betweenthe response characteristics of the respective tubes comprising acircuit connected between said control gridcircuits independently of thegrid-cathode lead,

tween the control grids of such tubes an auxiliary bias voltage thataids and opposes signal volt ages applied to the respective tubes.

9. The combination of claim 5 wherein the means for applying thepotential between the control grids is arranged to permit selection ofthe magnitude and reversal of the sense of such potential.

10. The combination of claim 6 wherein the means for applying thepotential between the control grids is arranged to permit selection ofthe magnitude and reversal of the sense of such potential.

11. In a dual tube-controlling grid circuit for applying signals ofopposite character to the control grids of a pair of electron tubes,means for compensating for variance between the sensitivitycharacteristics of response of such tubes to variations in magnitude ofsignal values of voltages applied to their respective control grids bysaid circuit, comprising a source of variable voltagecoupledeffectivelyin series between said grids and arranged to apply between them avoltage having a biasing characteristic for equally and oppositelyvarying the response characteristics of such tubes, and single controlmeans for simultaneously, equally and oppositely varying the biasingcharacteristic of such voltage effective on the control grids of suchtube and for selecting the sense of such voltage.

12. In a dual tube-controlling grid circuit for applying signals ofopposite character to the control grids of a pair of electron tubes,means in accordance with claim 11, wherein said source of variablevoltage comprises an impedance coupled effectively in series between thecontrol grids of said tube, means for applying voltage across saidimpedance, and adjustable control means arranged for selecting sense andmagnitude of voltage applied between such grids.

PAUL GLASS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

